4 replies to this topic

[aliadnan]

hi .. I want to know how high pressure is created in vessels or exchangers ... like we say we are producing 1500psi saturated steam in a heat exchanger or a vessel is operated at high pressure .. how pressure is created ... do we pump the fluid in the exchanger at 1500psi .. I want to know how high pressure steam is produced and how high pressure is created in vessels

[gvdlans]

Ideal Gas Law: p=nRT/V

So you can increase pressure of a gas by:
increasing n (increasing number of gas molecules in your volume)
increasing temperature
decreasing volume

Imagine you have a closed vessel, partly filled with water at atmospheric pressure. When you increase the temperature (e.g. by circulating part of the water through a fired heater), water will boil at 100°C. When you keep on heating the water, both pressure and temperature will increase (according to the boiling curve). If you continue doing this, at some point you reach a pressure of 100 bara (~1500 psia) and a temperature of 311°C. You can then remove the steam via a pressure control valve with a setting of 100 bara and you are producing high pressure steam.

[aliadnan]

thanks for your reply.. actually I was talking about a continuous process in which water (any fluid) entes the exchnger and during its passage from the excahnger it is converted into 1500psi steam and then leaves the exchanger ... Is/nt your reply related to a batch process in which we add water in a vessel and heat it untill it starts to boil and its pressure reaches 1500psi and when the pressure indicator indicates the required pressure the control valve opens ...
I have another question since there is 1500psi pressure inside the vessel or exchanger than we should be entering the water at pressure above 1500psi ?

[gvdlans]

When you start removing the steam through the pressure control valve you have to send in boiler feed water in order to make it a continuous process. This is normally done on level control. You also have to take out a small water flow (blowdown) to prevent build-up of contaminants. If the vessel is operated at 1500 psi, the boiler feed water needs to have a pressure above 1500 psi (since fluids flow from high to low pressure). So you need a pump that can supply sufficient head.

[Art Montemayor]

Aliadnan:

I complement you on your candor and straight-forwardness upon asking such a basic and important question. The subject of just exactly how pressure is produced out in industry is sometimes a perplexing one for engineering students. I understand. Far too often, engineering professors fail to revert back to basic physics and chemistry to explain the “when and whys” of a process and simply take it for granted that their students have a complete understanding and conceptualization of what he/she are talking about. And far too often, the attending student doesn’t have a complete understanding of what is really taking place in the subject process. And the damage is further complicated by the fact that far too few students will attempt the courage to ask the basic question: just how did the pressure appear and how is it sustained? The reason the average student doesn’t ask such questions, usually, is because of an underlying fear of being identified as ignorant of facts that should be well-known to him/her and this causes embarrassment as well as loss of self-esteem. This can do a lot of harm to a student’s capacity of taking interest and excelling at his engineering courses.

It has been my experience with young engineers that although they have been taught the basic science, many have not been taught how it is applied and used out in industry. Consequently, this has resulted in a void (or a discontinuity) in the young engineer’s ability to use his learning. One factor that has an influence on a student’s success is the ability of being able to conceptualize what is happening in a process. This doesn’t require a high IQ or superior intelligence; it’s an acquired skill or a gift – but it helps tremendously in being a successful engineer. It enables you to form your own mental “illustrations” or examples while you are listening to another person describe a process. A perfect example is one of VAPOR PRESSURE. As simple as it is, few engineers understand that everything in the universe has a vapor pressure and if left to Nature’s care, any substance (including steel) will strive to exert a vapor pressure on its surroundings. As simple as vapor pressure is, many engineers fail to understand the connection with PARTIAL PRESSURE and the important roles both play in the real world. Both these simple concepts form the basis for some very important processes and operations in the industrial world.

gvdlans has given you an excellent example and explanation of how steam pressure is generated within a process vessel. He has given you the same example of vapor pressure, applied in a steam generator as you requested. His response was directed exactly as you asked. He left the part of a steady-state steam generation continuing on to your ingenuity. If you can conceive a boiler creating steam at 1,500 psig, then you must also use your imagination and common sense to figure out that if you employ some of the steam outside the boiler, you must replenish the inventory that you export (“what goes in must come out”). It’s a material and energy balance – as we say in chemical engineering. And since you must sustain a steam pressure of 1,500 psig within the boiler, then you are forced to supply the make-up feed water at a supply pressure in excess of 1,500 psig to allow it to enter your thermodynamic system. You use a feedwater pump for that purpose and you can also return steam condensate to the same boiler. You’d be very foolish not to return the steam condensate to the boiler because of the cost of having to purify raw feed water and also because the condensate returns preheated. This is the way an engineer employs his common sense to put together a process.

Pressure within a process can be generated within the process or imported into the process. You just got a good example of how it is generated: apply a heat source and create a vapor pressure above a liquid source (as in your kettle steam generator). You can also create pressure by using a chemical reaction or a chemical decomposition within the process. The chemical reaction can be one where gaseous products are produced (as in a gasifier). A chemical decomposition is illustrated by the decomposition of Hydrogen Peroxide. These are common operations in some processing plants. You can also import the pressure by using gas compressors to mechanically compress a gas and feed the resultant high-pressure gaseous product to a process. Hydrogenation reactions are typical examples of this. The production of Coca-Cola relies on the ability to produce a carbonated water by subjecting the water to a high-pressure source of CO2 that goes into solution with the water(via Henry’s Law) and results in the carbonated product that everyone drinks. The carbonation process takes place on a continuous basis – you can appreciate just how continuous it is if you visit a friendly bottling plant nearby and witness all the bottles of beverage coming out of the bottling machine.

The reason I’m writing this is to give emphasis to the subject of producing process pressure. Sometimes it is a necessity – at other times, unfortunately, it is a very dangerous hazard. It is vitally important for a chemical engineering student to be aware of these basic facts. You will need to fully understand and conceptualize the phenomena of process pressure and how it is created or imported in order to be able to ensure a safe operation for you and the people who will eventually be working under your direction and orders in the future. If you haven’t done so already, you should visit and avidly read the threads in the Pressure Relief Devices Forum. There, you will find perfect examples of why a chemical engineer must dominate this knowledge. You must be prepared to identify the possibility and existence of any sudden or hazardous pressure situations within a process plant that can create a danger to you, your personnel, and your plant. Before you can design a protective system against a dangerous pressure, you must be able to understand and identify where and why the dangerous pressure appears. And deadly pressures don’t have to be 1,500 psig; you can get killed by 15 psig just as easily.

And there you have it. It is simply not enough to know how to create or import pressure; you must be able to control it 100% of the time. And I refer not only to the pressure that you directly create or import, but also to ANY pressure that may arise – either by accident or error – within your plant. You owe that to your well-being, your worker’s safety, and the investment that is placed in your care.

Keep asking questions – no matter how silly, embarrassing, or stupid they may seem. As long as the question is based on an engineering application and it is seriously asked, it won’t be wasted or treated as un-important - at least not by me. We all learn by asking questions.